The present invention relates to a substrate bias sputtering process capable of forming a film on a substrate having satisfactory coverage, and an apparatus for carrying out the same.
Recently, the sputtering process has been applied widely to forming metallic thin films and dielectric thin films for microelectronic parts. The sputtering process is explained in detail in "Fundamentals of Thin Film Formation" (T. Asamaki, "Sputtering Phenomena", Nikkan Kogyo Shinbun --sha, 1987). The sputtering process is practiced in various systems. The present invention relates to a bias sputtering process. The background of the bias sputtering process will be described hereinafter.
FIG. 2 is a diagrammatic illustration for aid in explaining the basic concept of the bias sputtering process. As shown in FIG. 2, a sputtering electrode 202 and a substrate electrode 203 are disposed within a vacuum vessel 201. A substrate 204 on which a film is to be formed is mounted on the substrate electrode 203. After evacuating the vacuum vessel 201 by suitable evacuating means, not shown, to a high vacuum in the range of 10.sup.-8 to 10.sup.-7 torr, a sputtering gas is introduced into the vacuum vessel 201 by suitable gas supply means, not shown, to maintain the internal gas pressure of the vacuum vessel 201 at several millitorrs. Ordinarily, the sputtering gas is a rare gas, such as argon gas.
The sputtering electrode 202 is electrically energized. That is, the sputtering electrode 202 is connected to a dc high-voltage power supply 206 or a radio frequency power supply, not shown, to charge the surface of a sputtering target 205 attached to the front surface of the sputtering electrode 202 at a high negative potential. Generally, the sputtering target 205 is formed of a film forming material which is to be deposited on the substrate 204. Generally, a dc power supply is used for forming metallic films.
Upon the application of a high voltage to the sputtering electrode 202, discharge occurs at the sputtering target 205 to produce a plasma 208 in front of the sputtering target 205. Sputtering gas ions, usually, argon ions, in the plasma are accelerated by the high negative potential of the sputtering target 205 to impinge on the sputtering target 205. The impact of the sputtering gas ions on the sputtering target 205 causes the sputtering target 205 to sputter the component material to deposit the material on the substrate 204.
The substrate electrode 203 is connected to a substrate bias power supply 207. The substrate electrode 203 and the substrate bias power supply 207 are provided for maintaining the surface of the substrate at a substantially negative potential. Although it is possible to use a dc power supply as the bias power supply 207 in forming a metallic film and to use a radio frequency power supply as the bias power supply 207 in forming an insulating film, in most cases, a radio frequency power supply, which is effective for both forming a metallic film and forming an insulating film, is employed as the bias power supply 207.
The purposes of applying a bias voltage to the substrate will be itemized hereunder.
1. Removal of gaseous impurities from the film during film formation.
2. Control and improvement of the properties, such as hardness and crystallinity, of the film.
3. Improvement of the adhesion of the film to the substrate.
4. Improvement of the conformance of the film to the surface configuration of the substrate.
Although the application of a bias voltage to the substrate is a common practice, the effect of bias voltage application depends on the material used and operating conditions of the apparatus. Therefore, the foregoing purposes do not necessarily apply universally.
The prior art relating to the improvement in the conformance of a film to the surface configuration of the substrate, which is one of the objects of the present invention, will be described hereinafter.
The microminiaturization of LSIs (large-scale integrated circuits) has been promoted increasingly, which has developed a multilayer wiring structure comprising alternate layers of wiring films and insulating films for the microminiaturization of the circuits of LSI chips. One of the principal problems in the industrial production of such a multilayer wiring structure is the connection of adjacent wiring layers insulated from each other by an insulating film. The upper wiring layer and the adjacent lower wiring layer are interconnected through through holes formed in the insulating layer interposed between the upper and lower wiring layers. The size, i.e., the diameter, of the through holes has been decreased with the increasing microminiaturization of the LSI.
FIG. 3 is a sectional view showing a structure for the interconnection of adjacent wiring layers through through holes 304. In this example, the wiring lines 301 of a first wiring layer are about 3 .mu.m in width and 1 .mu.m in height (thickness), the wiring pattern 303 of a second wiring layer is about 1 .mu.m in thickness, an insulating layer 302 interposed between the first and second wiring layers is 0.8 .mu.m in thickness, and the diameter of the through holes 304 is about 2 .mu.m. FIG. 4 shows a structure for a further minute wiring, in which wiring lines 401 of a first wiring layer are 2 .mu.m in width. To interconnect wiring lines having a width of 2 .mu.m, the diameter of the through holes 404 must be reduced accordingly. Therefore, the diameter of the through holes 404 of the structure shown in FIG. 4 is 1 .mu.m.
Currently, the wiring patterns of the LSI are formed of aluminum or an aluminum alloy and, in most cases, aluminum films are formed by a sputtering process. The sputtering process, as compared with the evaporation process, is capable of forming a film well conforming to the surface configuration of the substrate. However, in forming a film over an underlying layer having sharp irregularities and holes having a small solid angle limited by the shape of the substrate, the film deposited by the sputtering process has a problem in the conformance of the film to the irregular surface configuration.
The relation between the shape of the through hole and the conformance of the film to the through hole is represented by aspect ratio. Aspect ratio is the value obtained by dividing the depth of the through hole by the diameter of the same. The greater the aspect ratio, the sharper is the shape of the through hole. The aspect ratio of the through holes shown in FIG. 3 is 0.4 and that of the through holes shown in FIG. 4 is 1.0. The conventional sputtering process is able to coat the surface of through holes having an aspect ratio below about 0.5 without any significant problem. However, the covering performance of the film deposited by the conventional sputtering process becomes unsatisfactory as the aspect ratio increases to one, as shown in FIG. 4.
Problems in coating the surface of the through holes of the LSI of a multilayer wiring structure with an aluminum film is explained in detail, for example, in "Special Edition: Flattening the Multilayer Wiring Structure", Semiconductor World, No. 10, pp. 116-137, 1984.
Various trials including forming a moderately irregular surface of an underlying layer have been made to improve the covering performance of an aluminum film formed by a sputtering process. As mentioned above, there are many reports demonstrating the effectiveness of the bias sputtering process on the improvement of the covering performance of aluminum films. The principle of the bias sputtering process relating to the improvement of the covering performance will be described hereinafter.
When the surface of a substrate is maintained at a negative potential, argon ions of a plasma produced by the sputtering electrode are accelerated by the negative potential to impinge on the surface of the substrate. It is inferred that the energy of the argon ions produces various favorable effects. When the energy of the argon ions is sufficiently large, aluminum atoms in a film deposited on the surface of the substrate are caused to sputter again, thereby leaving the film. Consequently, aluminum atoms deposited on the corner of a through hole are liable to be caused to sputter again by the impact applied thereto by the argon ions. Thus, the repetitive resputtering of aluminum atoms deposited on the surface of the substrate during the film forming process forms a smooth and substantially flat aluminum film over the surface of the through hole.
On the other hand, although the aluminum atoms do not sputter again when the energy of the argon ions is insufficient, the argon ions cause thermal excitation of the aluminum atoms deposited on the surface of the substrate. Since the kinetic energy of the argon ions is greater than the bond energy of the aluminum atoms, the argon ions are able to rearrange the aluminum atoms in the deposited aluminum film, even if the argon ions are unable to cause the resputtering of the aluminum atoms.
Actually, it is possible that the argon ions provide the foregoing two effects simultaneously. Improvements in the covering performance of aluminum films are reported in David W. Skelly et al., "Significant Improvement in Step Coverage Using Bias Sputtered Aluminum", J. Vac. Sci. Technol. A4(3), pp. 457-460, May/June, 1986.
FIG. 5 shows the results of through hole coverage tests, in which the level of the dc substrate bias voltage applied to the substrate was varied in forming aluminum films of 1 .mu.m in thickness over the surfaces of through holes of 1 .mu.m in diameter and 1.3 .mu.m in depth to evaluate the effect of the dc substrate bias voltage on the through hole coverage of the aluminum films. In FIG. 5, indicated at 501 is the sectional shape of an aluminum film for a substrate bias voltage of -75 V, at 502 is the sectional shape of an aluminum film for a substrate bias voltage of -100 V, at 503 is the sectional shape of an aluminum film for a substrate bias voltage of -150 V, and at 403 is the sectional shape of an aluminum film when the substrate bias voltage is zero. The sectional shape 501 and the sectional shape 403 are substantially the same, which proves that the coverage improving effect of a substrate bias voltage of -75 V is insignificant. As is obvious from the sectional shape 502, the aluminum film conforms satisfactorily to the surface of the through hole and a substrate bias voltage of -100 V is effective for improvement of the coverage. The substrate bias voltage of 31 150 V deteriorates the through hole coverage of the aluminum film. It is considered to be due to the obstruction of the deposition of aluminum in the through hole by an overhang portion of the aluminum film around the edge of the through hole caused by an excessive rise in temperature of the aluminum film deposited around the through hole. In the worst case, the through hole is closed by the portion of the aluminum film overhanging the through hole to form a void 504 within the through hole, which greatly deteriorates the reliability of the through hole as means for interconnecting wiring patterns.
Thus, the bias sputtering process is able to form a film in satisfactory conformance to the configuration of a surface. However, the bias sputtering process has the following problems in its practical application.
1. The film formed by the bias sputtering process absorbs argon (sputtering gas). The argon absorption of the film increases with increase in the bias voltage. In the LSI manufacturing process, a LSI chip is subjected to an annealing process after the film forming process. The LSI chip is exposed to a high temperature in the range of 430.degree. to 475.degree. C. in the annealing process, whereby argon atoms contained in the aluminum film condense within the aluminum film and break weak portions of the aluminum film, thereby forming small holes of several microns to several hundreds of microns in the aluminum film. Such a tendency is more noticeable when the bias voltage is higher.
2. The application of a bias voltage increases the specific resistance of the film. For example, an aluminum film of 1 .mu.m in thickness formed by a sputtering process without using any bias voltage has a specific resistance on the order of 2.7 .mu..OMEGA..sup.-cm. However, the specific resistance increases with increase in the bias voltage. The electrical conductivity of a thin film is limited mainly by crystalline defects in the grain boundaries. Accordingly, it is appropriate to consider that a thin film having a high electrical resistance has many crystalline defects.
3. The aluminum film formed by the bias sputtering process has a comparatively low reflectance. Power is supplied to the substrate owing to the substrate bias voltage and the resulting current to increases the temperature of the substrate excessively during the film forming, process. As a result, excessively coarse grains are formed, thereby reducing the specular reflectance. The specular reflectance is a significant factor affecting conditions for the exposure of a photoresist in the following patterning process, and hence the specular reflectance must be higher than a certain level dependent on the wavelength of light used for exposure.
Generally, in the process of growth of an aluminum thin film, it is considered that, initially, aluminum nuclei are formed on the surface of a substrate, the aluminum nuclei grow to form individual island structures (up to 80 .ANG.), and then the individual island structures join together to form a continuous, uniform film (up to 200 .ANG.). Thus, the quality of the thin film is affected by all the film forming conditions from the initial stage of the film forming process before the continuous thin film is formed.
On the other hand, as mentioned above with reference to the conventional techniques, it is possible to improve the crystallinity of the thin film and the adhesion of the same to the substrate by applying a bias voltage to the substrate. The application of the bias voltage to the substrate at the initial stage of the film forming process, in which the aluminum nuclei are formed, is particularly effective for forming a thin film having improved quality. In a stage immediately after the start of the film forming process (up to 200 .ANG.), the nuclei are formed in individual island structures and hence the underlying insulating film is partly exposed; consequently, the dc bias sputtering process is unable to apply a dc bias voltage uniformly to the surface of the substrate on which the aluminum is to be deposited. Accordingly, the radio frequency biasing method capable of applying a bias voltage to the surface of the substrate through the insulating film is advantageous over the dc biasing method. However, when radio frequency power is applied to the substrate for biasing the substrate, argon ions having comparatively high energy impinge on the substrate, which enhances the aforesaid problems in the practical application of the dc bias sputtering process